Generally, under a wireless channel environment, information is lost because an inevitable error occurs due to various factors such as multi-path interference, shadowing, attenuation of the radio wave, time-varying noise, and fading. The loss of information acts as a factor that severely distorts an actually transmitted signal and thus degrades the entire performance of a wireless communication system.
A space diversity technique may be used for preventing the performance of a communication system from being degraded due to fading among the various factors. In the space diversity technique, a spatial domain for use of resources is additionally secured by disposing a plurality of antennas in a transmitting end and a receiving end. Therefore, the reliability of a communication link increases with a diversity gain without the increase in a bandwidth, and a transmission rate can increase through parallel transmission based on spatial multiplexing.
In a wireless communication system, a Single-Input Single-Output (SISO) system and an MIMO system are being generally used for transmitting data.
The SISO system is technology that transmits data over one channel (H) that is established between one transmitting antenna (TxAnt) and one receiving antenna (RxAnt), and since a transmitting end and a receiving end communicates with each other through one antenna, multi-path interference or fading occurs on a transmission path due to an obstacle such as a hill or a steel tower. Due to this reason, a data transmission speed decreases or a data transmission error increases.
Unlike this, in the MIMO system, a plurality of antennas are disposed in a transmitting end and a receiving antenna for transmitting data through several paths. The receiving end decreases interference because detecting a signal received through each path, and the transmitting end increases transmission efficiency through space-time diversity and spatial multiplexing. As an example, a 2×2 MIMO system uses two transmitting antennas and two receiving antennas. In such a 2×2 MIMO system, four channels (H11, H12, H21, and H22) are established between first and second transmitting antennas (TxAnt1 and TxAnt2) and first and second receiving antennas (RxAnt1 and RxAnt2).
An MIMO mode usable for the MIMO system is categorized into a Spatial Diversity (SD) mode and a Spatial Multiplexing (SM) mode according to a scheme of allocating a plurality of antennas and symbols to be transmitted, and particularly, a wireless communication system based on Institute of Electrical and Electronics Engineers (IEEE) 802.16 defines the selective use of the SD mode and SM mode. Herein, the SD mode denotes a technique that transmits the same data through a plurality of antennas, and the SM mode denotes a technique that transmits different data by antenna.
In determining an MIMO mode for a mobile station, the related art switches an MIMO mode of the mobile station from the SD mode to the SM mode when a Carrier to Interference Noise Ratio (CINR) value transmitted from the mobile station is greater than or equal to a threshold value, but when a Modulation and Coding Scheme (MCS) level of the mobile station is less than or equal to a threshold MCS level, the related art switches the MIMO mode of the mobile station from the SM mode to the SD mode.
Therefore, in a mobile station that has a CINR value greater than a threshold value in spite of low decoding capability, since the CINR value of the mobile station is greater than or equal to the threshold value when an MIMO mode of the mobile station is the SD mode, the related art switches the MIMO mode of the mobile station from the SD mode to the SM mode. However, in the SM mode, since the mobile station cannot normally decode received data because due to low decoding capability, an MCS level of the mobile station decreases inevitably, and thus, the related art again switches the MIMO mode of the mobile station from the SM mode to the SD mode.
Subsequently, the related art again switches the MIMO mode from the SD mode to the SM mode due to the CINR value in the SD mode, and again switches the MIMO mode from the SM mode to the SD mode because the MCS level decreases due to low decoding capability in the SM mode. Such an operation is repeatedly performed, and consequently, the MIMO mode of the mobile station is switched frequently.
In addition to such limitations, since the related art determines an MIMO mode of a corresponding mobile station with no consideration of frequency efficiency in each MIMO mode, communication efficiency is reduced.